Array of geiger-mode avalanche photodiodes for detecting infrared radiation

ABSTRACT

An array of Geiger-mode avalanche photodiodes is formed in a die and includes: an internal dielectric structure, arranged on the die; and an external dielectric region arranged on the internal dielectric structure. The external dielectric region is formed by an external material that absorbs radiation having a wavelength that falls in a stop-band with low wavelength and transmits radiation having a wavelength that falls in a pass-band with high wavelength, at least part of the pass-band including wavelengths in the infrared. The internal dielectric structure is formed by one or more internal materials that substantially transmit radiation having a wavelength that falls in the stop-band and in the pass-band and have refractive indices that fall in an interval having an amplitude of 0.4. In the stop-band and in the pass-band the external dielectric region has a refractive index with the real part that falls in the above interval.

BACKGROUND Technical Field

The present disclosure relates to an array of Geiger-mode photodiodesfor detecting infrared radiation.

Description of the Related Art

As is known, today sensors for detecting infrared radiation (i.e.,radiation with a wavelength of between 700 nm and 2000 nm) findwidespread use not only in the sector of telecommunications, but also,for example, in the sector of so-called 3D imaging, as well as, onceagain by way of example, in the sector of thermography andplethysmography.

Currently used for detection of infrared radiation are, among otherthings, semiconductor sensors operating in the linear region, avalanchephotodetectors, phototransistors, and vacuum photomultiplier tubes.Instead, the so-called arrays of Geiger-mode avalanche photodiodes(GMAPs) are relatively little used in the field of detection of infraredradiation, since, even though they guarantee a good sensitivity inregard to infrared radiation, they are subject to dark currents thatincrease considerably as the reverse biasing voltage increases, withconsequent reduction of the signal-to-noise ratio.

In greater detail, a Geiger-mode avalanche photodiode, also known assingle-photon avalanche diode (SPAD), is formed by an avalanchephotodiode (APD), and thus comprises a junction of semiconductormaterial, which has a breakdown voltage V_(B) and is biased, in use,with a reverse biasing voltage V_(A) higher in modulus than thebreakdown voltage V_(B), which, as is known, depends upon thesemiconductor material and upon the doping level of the least dopedregion of the junction itself. In this way, the junction has aparticularly extensive depleted region, present inside which is anon-negligible electrical field. Thus, generation of a singleelectron-hole pair, following upon absorption of a photon incident onthe SPAD, may be sufficient to trigger an ionization process. Thisionization process in turn causes an avalanche multiplication of thecarriers, with gains of around 10⁶ and consequent generation in shorttimes (hundreds of picoseconds) of the avalanche current, or moreprecisely of a pulse of the avalanche current.

The avalanche current may be appropriately collected, typically by meansof external circuitry connected to the junction, and represents anoutput signal of the SPAD, which will be referred to also as outputcurrent. In practice, for each photon absorbed, a pulse of the outputcurrent of the SPAD is generated.

The fact that the reverse biasing voltage V_(A) is higher than thebreakdown voltage V_(B) causes the avalanche ionization process, oncetriggered, to be self-sustaining. Consequently, once the ionizationprocess is triggered, the SPAD is no longer able to detect photons. Tobe able to detect also the subsequent photons, the avalanche ionizationprocess must be stopped, by lowering, for a period of time known as“hold-off time”, the effective voltage across the junction. For thispurpose, it is known to use of so-called quenching circuits, which maybe either of an active type or of a passive type. For example, in thecase of passive quenching, the quenching circuits may be formed byintegrated resistors.

This being said, an array of SPADs is formed by a planar array of SPADsgrown on one and the same substrate. The anode and cathode electrodes ofthe SPADs may be respectively connected together so that the SPADs maybe biased at one and same reverse biasing voltage V_(A), in which casethe array forms a so-called silicon photomultiplier (SiPM). Further, inthe case of a SiPM, the SPADs are provided with respective quenchingresistors (for example, of a vertical type), which are integrated in theSPADs and are decoupled from one another and independent. In addition,the avalanche currents generated within the SPADs are multiplexedtogether for generating an output signal of the SiPM equal to thesummation of the output signals of the SPADs, the output signal beingthus proportional to the number of photons that impinge upon the SiPM.

In general, any array of SPADs is affected by the phenomenon of opticalcrosstalk.

In detail, given any SPAD of an array, the corresponding operation isaffected by the photons generated by electroluminescence duringavalanche multiplication processes triggered in surrounding SPADs.

In greater detail, it is known that the SPADs operating above thebreakdown voltage emit in an isotropic way secondary photons, on accountof various mechanisms such as, for example, (direct and indirect)interband recombinations and direct intraband electronic transitions.The secondary photons are generally emitted within a wavelength rangecomprised between 400 nm and 2 μm, with a likelihood of emission thatdepends upon the reverse biasing voltage V_(A) applied.

The secondary photons may propagate and be subsequently absorbed in thejunctions of SPADs different from the SPADs in which they have beengenerated, triggering avalanche events that cause the aforesaid opticalcrosstalk.

This being said, assuming, for example, an array of SPADs biased withone and the same reverse biasing voltage V_(A), as the overvoltageOV=−(V_(A)−V_(B)) increases, there occurs an increase in the gain and inthe detection efficiency of the array. However, there further occurs, asmentioned previously, an undesirable increase in the dark current, whichis triggered by spurious events such as optical crosstalk andafterpulsing. For this reason, in the context of detection of infraredradiation there are used at most arrays biased with low reverse voltages(typically, such that the overvoltage OV will be around 2 or 3 V).Further, the arrays used in the context of detection of infraredradiation have limited sensitive areas (typically, in the region of afew square millimeters) since the dark current increases as thesensitive area increases.

BRIEF SUMMARY

At least one embodiment of the present disclosure is an array ofGeiger-mode avalanche photodiodes that enables the drawbacks of theknown art to be overcome at least in part.

According to the disclosure, an array of Geiger-mode avalanchephotodiodes is provided as defined in the annexed claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the disclosure, embodiments thereof arenow described, purely by way of non-limiting example and with referenceto the attached drawings, wherein:

FIG. 1 is a schematic perspective view of an array of photodiodes;

FIG. 2 is a schematic cross-sectional view of a photodiode of an arrayof photodiodes;

FIG. 3a is a schematic perspective view from above of an array ofphotodiodes with portions removed;

FIG. 3b is a schematic perspective view from beneath of the array ofphotodiodes shown in FIG. 3 a;

FIG. 4 is a schematic cross-sectional view of two adjacent photodiodesof an array of photodiodes;

FIG. 5 is a schematic cross-sectional view of two photodiodes belongingto two respective adjacent arrays of an array of arrays of photodiodes;and

FIGS. 6 and 7 show block diagrams of systems that use an array ofphotodiodes.

DETAILED DESCRIPTION

FIG. 1 shows an embodiment of an array 99 of SPADs 1, which are arrangedin planar fashion and are illuminated by a light source 200. The array99 may comprise any number of SPADs 1, according to the need. Further,the array 99 is integrated in a die 100, which, as shown in FIG. 2(which is not in scale), comprises a semiconductor body 10 made, forexample, of silicon.

Purely by way of example, the semiconductor body 10 comprises asubstrate 2, which is delimited at the bottom and at the top by a bottomsurface S_(inf) and a top surface S_(sup), respectively. Further, thesemiconductor body 10 comprises a first epitaxial layer 4 and a secondepitaxial layer 8.

The substrate 2 is of an N++ type, has a thickness, for example, ofbetween 300 μm and 500 μm and has a doping level, for example, ofbetween 1·10¹⁹ cm⁻³ and 5·10²⁰ cm⁻³. Further, the substrate 2 forms theaforementioned bottom surface S_(inf).

The first epitaxial layer 4 is of an N−− type, has a thickness, forexample, of between 5 μm and 10 μm and overlies the substrate 2, indirect contact therewith. Further, the first epitaxial layer 4 has adoping level, for example, of between 1·10¹⁴ cm⁻³ and 5·10¹⁴ cm⁻³.

The second epitaxial layer 8 is of an N− type, has a thickness, forexample, of between 2 μm and 5 μm and overlies the first epitaxial layer4, in direct contact therewith. Further, the second epitaxial layer 8forms the aforementioned top surface S_(sup). The doping level of thesecond epitaxial layer 8 is, for example, of between 5·10¹⁵ cm⁻³ and1·10¹⁶ cm⁻³.

The SPADs 1 are, for example, the same as one another. Thus, in whatfollows just one of the SPADs 1, included in a packaged optoelectronicdevice 150 shown in FIG. 2, is described by way of example.

In detail, the SPAD 1 comprises an anode region 12, which is of a P+type and has, in top plan view, a circular or polygonal (for example,quadrangular) shape. The anode region 12 extends in the second epitaxiallayer 8 from the top surface S_(sup). In particular, the anode region 12has a thickness, for example, of between 0.1 μm and 0.5 μm. Further, theanode region 12 has a doping level, for example, of between 1·10¹⁹ cm⁻³and 1·10²⁰ cm⁻³.

The SPAD 1 further comprises an enriched region 14 of an N+ type, whichextends in the second epitaxial layer 8, underneath, and in directcontact with, the anode region 12. In top plan view, the enriched region14 has a circular or polygonal (for example, quadrangular) shape.Further, the enriched region 14 has a thickness of, for example, 1 μmand a doping level, for example, of between 1.10¹⁶ cm⁻³ and 5.10¹⁶ cm⁻³.

For practical purposes, the anode region 12 and the enriched region 14form a first PN junction designed to receive photons and to generate theavalanche current, as described in detail hereinafter. In other words,the anode region 12 and the enriched region 14 are in contact with oneanother along an interface surface I.

The enriched region 14 and the second epitaxial layer 8 have, instead,the purpose of confining a high electrical field in the proximity of thefirst PN junction, reducing the breakdown voltage V_(B) of the junctionitself.

The SPAD 1 further comprises a guard ring 16 of a circular shape, of aP− type and with a doping level of between 1·10¹⁶ cm⁻³ and 3·10¹⁶ cm⁻³.In particular, the guard ring 16 extends in the second epitaxial layer 8from the top surface S_(sup). Further, the guard ring 16 surrounds theanode region 12, contacting a peripheral portion of the latter. Inaddition, the guard ring 16 has a thickness, for example, of between 1μm and 3 μm.

The guard ring 16 forms a second PN junction with the second epitaxiallayer 8 for preventing edge breakdown of the anode region 12. Further,the guard ring 16 is in direct electrical contact with an anodemetallization 18, which is arranged over the top surface S_(sup) andenables biasing of the first PN junction. In particular, in use, it ispossible to apply to the anode metallization 18 a reverse biasingvoltage V_(A) higher, in modulus, than the breakdown voltage V_(B) ofthe first PN junction. Albeit not shown in FIG. 2, the anodemetallization 18 is connected in a way in itself known to conductivepads 19 (shown in FIG. 3a ) present on the die 100, which will bereferred to in what follows as the die pads 19.

The SPAD 1 further comprises a lateral insulation region 24, whichsurrounds, at a distance, the guard ring 16 and performs the function ofpreventing secondary photons generated in the SPAD 1 from being absorbedby adjacent SPADs. The lateral insulation region 24 is formed within acorresponding trench 29 having an annular shape, in top plan view.

In detail, the lateral insulation region 24 extends within thesemiconductor body 10, starting from the top surface S_(sup). In topplan view, the lateral insulation region 24 has a circular or polygonalshape and delimits laterally the active area A of the photodiode 1.

In greater detail, the lateral insulation region 24 comprises achannel-stopper region 27, arranged externally, and a barrier region 28,arranged internally.

The channel-stopper region 27 is made of dielectric material (forexample, oxide) and is arranged in direct contact with the semiconductorbody 10.

The barrier region 28 is made of polysilicon and is surrounded by thechannel-stopper region 27, with which it is in direct contact. Further,the barrier region 28 is in direct contact with a dielectric layerdescribed in detail hereinafter and referred to as the fourth dielectriclayer 40.

In greater detail, the polysilicon that forms the barrier region 28 mayhave a doping either of an N type or of a P type and has a doping level,for example, of between 1·10¹⁹ cm⁻³ and 5·10²⁰ cm⁻³.

In yet greater detail, the lateral insulation region 24 extends in thesemiconductor body 10 starting from the top surface S_(sup) fortraversing the first and second epitaxial layers 4, 8, as well as toextend in a top portion of the substrate 2.

Purely by way of example, the barrier region 28 has a width of, forinstance, 1 μm.

The array 99 further comprises a first dielectric layer 30, a seconddielectric layer 32, and a third dielectric layer 38, in addition to theaforementioned fourth dielectric layer 40, which are now described withreference to the corresponding arrangements within the SPAD 1.

In detail, the first dielectric layer 30 is formed, for example, bythermal oxide and extends over a portion of the top surface S_(sup)laterally staggered with respect to the anode region 12 for leaving theanode region 12 and a portion of the guard ring 16 that contacts theanode region 12 exposed.

The second dielectric layer 32 is formed, for example, by TEOS oxide andextends over the first dielectric layer 30, with which it is in directcontact, as well as over the anode region 12, with which it is in directcontact. Further, the second dielectric layer 32 extends also over theaforementioned portion of the guard ring 16 that contacts the anoderegion 12.

The second dielectric layer 32 functions as passivation region and maybe sized for forming an anti-reflection layer that increases opticaltransmission in the infrared.

The third dielectric layer 38 is formed, for example, by TEOS oxide,forms a single layer with the channel-stopper region 27, and extendsover the second dielectric layer 32, with which it is in direct contact,without, however, overlying a central portion of the anode region 12. Inother words, the third dielectric layer 38 is laterally staggered withrespect to the anode region 12 and to the underlying enriched region 14.

The fourth dielectric layer 40 is formed, for example, by TEOS oxide andextends over the third dielectric layer 38, with which it is in directcontact. Further, the fourth dielectric layer 40 extends for closing thelateral insulation region 24 at the top. In particular, the fourthdielectric layer 40 extends as far as into contact with the barrierregion 28.

The anode metallization 18 traverses the second, third, and fourthdielectric layers 32, 38, 40 for contacting the guard ring 16, asmentioned previously.

A cathode metallization 42, made of metal material, extends underneaththe bottom surface S_(inf) of the substrate 2, with which it is indirect contact. In this way, given the arrangement of the anodemetallization 18, the avalanche current generated by the SPAD 1 flows inthe direction of an axis H perpendicular to the bottom surface S_(inf)and to the top surface S_(sup).

For practical purposes, the enriched region 14, the substrate 2, and thefirst and second epitaxial layers 4, 8 form a cathode region of the SPAD1. Further, within the substrate 2, the voltage drop due to passage ofthe avalanche current is negligible, on account of the low resistivityof the substrate 2.

In addition, albeit not shown, the anode metallization 18 contacts acorresponding polysilicon region (not shown), which functions asquenching resistor. The polysilicon regions (not shown) of the SPADs 1are connected to a shared metal bus (not shown).

Since the lateral insulation region 24 extends as far as the substrate2, and given the low resistivity of the substrate 2, turning-on of oneSPAR 1 does not alter, to a first approximation, biasing of the adjacentSPADs 1. Consequently, the array 99 of SPADs 1 forms a semiconductorphotomultiplier, namely a SiPM, in which the SPADs 1 may worksubstantially in the same operating conditions. In this connection,albeit not shown, the anode and cathode metallizations of the SPADs 1 ofthe array 99 are configured so that they may all be connected to asingle voltage generator, which supplies the reverse biasing voltageV_(A).

The optoelectronic device 150 also includes packaging 152, such as of asurface mount device (SMD) type, as shown in FIGS. 2, 3 a, 3 b.

The packaging 152 comprises a lead frame 154 made of conductive material(for example, gold), which in turn comprises one or more pads 156 thatcontact the cathode metallization 42, as shown in FIGS. 3a and 3b .These pads 156 form corresponding contact terminals of the packageddevice 150, which are designed to enable biasing of the cathodemetallization 42.

The lead frame 154 further comprises one or more pads 158, each of whichis connected to a corresponding die pad 19 by means of a correspondingwire bonding 159. The pads 158 may thus be used for biasing the anodemetallizations 18 of the SPADs 1.

The packaging 152 further comprises a first packaging layer 160 (FIGS.2, 3 a, 3 b), which surrounds the die 100 laterally and at the top.

In greater detail, the first packaging layer 160 is formed by an epoxyresin, which is substantially transparent to radiation having awavelength comprised in the interval [400 nm, 1600 nm]. For example, theportion of first packaging layer 160 that overlies the die 100 transmitsat least 90% of the incident radiation having a wavelength comprised inthe interval [400 nm, 1600 nm].

Further, the first packaging layer 160 overlies the SPADs 1 and thusextends in contact with the fourth dielectric layer 40 and with thesecond dielectric layer 32 (where exposed, i.e., over the anode region12), as shown in FIG. 2. The first packaging layer 160 is delimited atthe top by a front surface S_(front).

The packaging 152 further comprises a second packaging layer 162 and athird packaging layer 164 (visible in FIG. 2, but not shown, forsimplicity, in FIGS. 3a and 3b ).

In detail, the second packaging layer 162 is arranged on the frontsurface S_(front), in contact with the first packaging layer 160. Thethird packaging layer 164 is arranged above the second packaging layer162, with which it is in direct contact.

In greater detail, the second packaging layer 162 is formed, forexample, by an optical glue (for example, a silicone glue) and isdesigned to glue the third packaging layer 164 to the first packaginglayer 160. Also the optical glue is substantially transparent toradiation having a wavelength comprised in the interval [400 nm, 1600nm]. For example, the second packaging layer 162 transmits at least 90%of the incident radiation having a wavelength comprised in the interval[400 nm, 1600 nm].

The third packaging layer 164 is made of a homogeneous material suchthat the third packaging layer 164 blocks, by absorbing it, radiationhaving a wavelength lower than λ_(filter)=700 nm, whereas it is to afirst approximation transparent to radiation having a wavelength higherthan λ_(filter). For example, the third packaging layer 164 may transmitat least 90% of the incident radiation having a wavelength higher thanκ_(filter) and may absorb at least 90% of the radiation having awavelength lower than λ_(filter)−Δ, where, for example, Δ=100 nm. Inother words, the third packaging layer 164 functions as single-bandhigh-pass optical filter.

In greater detail, the third packaging layer 164 is formed, for example,by allyl diglycol carbonate or polyallyl diglycol carbonate (also knownas CR39), in which case it functions as polymeric-plastic filter, with aresponse of a high-pass type.

In yet greater detail, it is possible to define a wavelengthλ_(cutoff)=1240/E_(g), where E_(g) (expressed in electronvolts) is theso-called energy gap of the semiconductor material that forms thesemiconductor body 10; for example, in the case of silicon,λ_(cutoff)≈1100 nm. This being said, we have λ_(cutoff)>λ_(filter).Further, the array 99 is designed to detect with particular efficiencyinfrared radiation having a wavelength comprised in the interval[λ_(cutoff), λ_(filter)].

From an optical standpoint, each of the first, second, and thirdpackaging layers 160, 162, 164 is homogeneous. Further, the first,second, and third packaging layers 160, 162, 164 have substantially oneand the same refractive index, for example of approximately 1.5. Forinstance, the refractive indices of the first, second, and thirdpackaging layers 160, 162, 164 are comprised in an interval [n_(inf),n_(sup)] having a width, for example, not greater than 0.4, preferablythan 0.2.

More precisely, it is assumed that in the interval [400 nm, 1600 nm] therefractive indices of the first and second packaging layers 160, 162have negligible imaginary components that are comprised in the interval[n_(inf), n_(sup)]. It is further assumed that in the interval [400 nm,1600 nm] the refractive index of the third packaging layer 164 has areal part comprised in the interval [n_(inf), n_(sup)]. Further, therefractive index of the third packaging layer 164 has a non-negligibleimaginary component for λ<λ_(filter)−Δ.

Without any loss of generality, it is assumed that, in the interval [400nm, 1600 nm], also the refractive index of the second dielectric layer32 is substantially real and is comprised in the interval [n_(inf),n_(sup)].

From an optical standpoint, the structure formed by the first, second,and third packaging layers 160, 162, 164 may be considered equivalent toa substantially homogenous structure. Consequently, it may be assumedthat at the interfaces present between the first, second, and thirdpackaging layers 160, 162, 164 there occur no phenomena ofreflection/refraction.

This being said, the presence of the structure formed by the first,second, and third packaging layers 160, 162, 164 does not modify theperformance of the array 99 in detection of infrared radiation. Rather,this structure contributes to filtering environmental light having awavelength different from the wavelengths of interest.

In addition, the presence of the structure formed by the first, second,and third packaging layers 160, 162, 164 enables improvement of theperformance of the array 99 in terms of dark current, as explainedhereinafter. In particular, in what follows reference is made to FIG. 4,where purely by way of example a portion of the array 99 is shown, whichincludes a first SPAD and a second SPAD adjacent to one another anddesignated, respectively, by 1A and 1B. For simplicity ofrepresentation, in FIG. 4 not all the references mentioned previouslyare shown.

In detail, it is assumed that the first SPAD 1A emits secondary photons(represented with a dashed line) having a first wavelength, for example,of between 400 nm and 600 nm, and secondary photons (represented with asolid line) having a second wavelength of between 700 nm and 1100 nm. Itis further assumed that these secondary photons are directed towards thefirst, second, and third packaging layers 160, 162, 164 for notintercepting the lateral insulation region 24 of the first SPAD 1A.Consequently, in principle, these secondary photons could cause opticalcrosstalk in the second SPAD 1B. In particular, in principle thesecondary photons having the first wavelength potentially represent themain cause of optical crosstalk in the second SPAD 1B, because, eventhough they are emitted in a smaller amount than the secondary photonshaving the second wavelength, they present a low likelihood of beingre-absorbed before coming out of the semiconductor material that formsthe first SPAD 1A, on account of the surface arrangement of theaforementioned first PN junction of the first SPAD 1A. Further, onceagain in principle, the secondary photons having the first wavelengthhave a greater likelihood of triggering avalanche events in the secondSPAD 1B, since the SPADs of the array 99 have a high detectionefficiency in the visible.

This being said, it may be assumed that the secondary photons having thefirst wavelength traverse the first and second packaging layers 160, 162without undergoing either reflection or refraction, until they penetrateinto the third packaging layer 164, where they are absorbed.Consequently, unlike what occurs in packagings of a traditional type,without (among other things) the second and third packaging layers 162,164, the secondary photons having the first wavelength are unable toreach the second SPAD 1B.

As regards, instead, the secondary photons having the second wavelength,also these reach as far as the third packaging layer 164 withoutundergoing either reflection or refraction. Then, these photons traversethe third packaging layer 164 until they impinge upon the interfacepresent between the third packaging layer 164 and the externalenvironment, where they may be reflected. Consequently, at least part ofthe photons having the second wavelength may traverse again the secondand first packaging layers 162, 160, until they penetrate into thesemiconductor material of the second SPAD 1B. However, the photons thatmanage to penetrate into the semiconductor material of the second SPAD1B have in any case a low likelihood of triggering spurious avalancheevents on account of their high depth of penetration into thesemiconductor material and of the relatively low detection efficiency ofthe SPADs at the second wavelength. For these reasons, the opticalcrosstalk between the SPADs of the array 99 is reduced, and consequentlythe array 99 is characterized by a reduced dark current even in thepresence of high overvoltages, without the area of the array 99 itselfbeing limited.

Incidentally, it should be noted how the possible use, instead of thethird packaging layer 164, of an interferential infrared high-passfilter would cause a considerable increase in the dark current since theaforementioned secondary photons having the first wavelength could bereflected towards the second SPAD 1B.

FIG. 5 shows an array 300 of arrays which comprise, purely by way ofexample, a first array 399 a and a second array 399 b. In particular, inFIG. 5 the first and the second SPADs, designated once again by 1 a and1 b, belong, respectively, to the first and second arrays 399 a, 399 b.

In greater detail, the packaging 152 encloses the array 300.

More in particular, the first packaging layer 160 overlies the SPADs ofthe first and second arrays 399 a, 399. Further, the packaging 152comprises, instead of the third packaging layer 163, a first externallayer 165 a and a second external layer 165 b, which form a sort of pairof portions of an external coating region and are juxtaposed withrespect to one another. The first and second external layers 165 a, 165b are fixed to corresponding underlying portions of the first packaginglayer 160. Further, to a first approximation, the first and secondexternal layers 165 a, 165 b have one and the same thickness.

In yet greater detail, the first and second external layers 165 a, 165 bare made of materials such that: i) the first external layer 165 aabsorbs radiation having a wavelength lower than λ_(filter1) and is to afirst approximation transparent to radiation having a wavelength higherthan λ_(filter1) and ii) the second external layer 165 b absorbsradiation having a wavelength lower than λ_(filter2) and is to a firstapproximation transparent to radiation having a wavelength higher thanλ_(filter2), with λ_(filter2)≠λ_(filter1). In addition, the first andsecond external layers 165 a, 165 b have substantially the samerefractive index as the first and second packaging layers 160, 162.

In practice, the array 300 is provided with two different detectioncapacities, in two different frequency bands, and may provide the sameadvantages described with reference to the array 99.

The optoelectronic device 150, including the array 99 and/or the array300, may be used in a generic detection system 500, shown in FIG. 6, inwhich the light source 200 illuminates the optoelectronic device 150,which is biased by a power supply 510 and is connected to amicrocontroller unit 520, possibly by interposition of apre-amplification stage (not shown). The microcontroller unit 520processes the output signal of the optoelectronic device 150 andsupplies a processed signal to a processor 530, which enables analysisof this processed signal and display of the information associated tothis processed signal on a display 540. In a way in itself known andthus not shown, the microcontroller unit 520 may comprise ananalog-to-digital converter and a microcontroller, arranged downstreamof the analog-to-digital converter.

As shown in FIG. 7, instead of the microcontroller unit 520 there may bepresent: a discriminator 550, which receives the output signal of theoptoelectronic device 150 and generates a pulse whenever said outputsignal exceeds a threshold, thus filtering the spurious events; and acounter 560, designed to increment a count at each pulse generated bythe discriminator 550, the value of the count being communicated to theprocessor 530.

The advantages that the present array of SPADs affords emerge clearlyfrom the foregoing discussion. In particular, the present array of SPADsis characterized by a higher signal-to-noise ratio (SNR) than avalanchedetectors of infrared radiation. Further, the present array of SPADs maypresent a large sensitive area and may be biased for presenting highovervoltages, with consequent improvement of the gain and of thedetection efficiency.

In addition, in the presence of luminous fluxes of low intensity, thepresent array of SPADs exhibits an extremely linear response, since justa few cells (i.e., SPADs) are turned on as a result of spuriouscrosstalk effects.

Finally, it is evident that modifications and variations may be made tothe photodiode described, without thereby departing from the scope ofthe present disclosure.

For example, instead of silicon, there may be present a differentsemiconductor material (for instance, germanium, silicon-germanium,arsenic-gallium-indium, etc.). Further, the types of doping, P and N,may be reversed.

In general, given a semiconductor material that forms the semiconductorbody 10 and the energy gap of which corresponds to a given wavelengthλ_(cutoff) that falls in the infrared region, the third packaging layer164 is made of any material that: i) absorbs radiation having awavelength λ<λ_(filter)−Δ and is transparent to radiation having awavelength λ>λ_(filter), with λ_(cutoff)>λ_(filter); and ii) hasrefractive index with a real part that falls in the interval [n_(inf),n_(sup)], in the wavelength range [λ*, λ_(cutoff)], with λ*<λ_(filter)Δ.As mentioned previously, in the interval [λ*, λ_(cutoff)] the refractiveindices of the first and second packaging layers 160, 162 havenegligible imaginary parts, and real parts that fall in the interval[n_(inf), n_(sup)].

Purely by way of example, in the case of germanium and of indiumarsenide, we have, respectively, λ_(cutoff)≈1850 nm and λ_(cutoff)≈3440nm. It is thus possible, for example, to make the semiconductor body ofgermanium and adopt a third packaging layer such that λ_(filter)=1400nm, for forming an array with excellent possibilities of application inthe telecommunications field, where radiation having a wavelength of1550 nm is frequently used. Other examples of materials that may be usedto form the semiconductor body are: silicon and germanium alloys;arsenic and gallium alloys; indium, gallium, and arsenic alloys;phosphorus and indium alloys; and cadmium, mercury, and telluriumalloys.

Irrespective of the semiconductor material chosen, the structure of thesemiconductor body 10 may be different from the one described, forexample, as regards the number, arrangement, and characteristics of theepitaxial layers, in addition, as has already been said, to thesemiconductor material that forms it.

More in general, the SPADs 1 of the array 99 may be different from whathas been described.

For example, the anode region 12, instead of giving out onto the topsurface S_(sup), may be overlain by a top region of an N type. In thiscase, the first PN junction is at a greater depth than what has beendescribed previously.

The guard ring 16 and/or the enriched region 14 may be absent, or in anycase may have different characteristics from what has been described.

The lateral insulation region 24 may differ from what has beendescribed, both as regards the geometry and as regards the structure andthe materials. For example, the channel-stopper region 27 may include aplurality of dielectric layers. Once again purely by way of example, thebarrier region 28 may be made of a material designed to reflect thesecondary photons (for example, tungsten), or else absorb the secondaryphotons (for example, the aforementioned polysilicon, or else titaniumnitride).

Possibly, the lateral insulation region 24 may be absent.

It is further possible for the SPAD 1 to be configured to implementquenching mechanisms different from the ones described. For example,each SPAD 1 may implement a corresponding integrated quenching resistor,of a vertical type, in which case the aforementioned polysilicon regionsare absent.

In addition, the number, geometrical characteristics, and materials ofthe dielectric layers arranged over the top surface S_(sup) may varywith respect to what has been described. For example, the seconddielectric layer 32, or in any case the portion of second dielectriclayer 32 that overlies the anode region 12 may be absent.

It is further possible for the second dielectric layer 32 (which, on theother hand, is optional) to have a refractive index different from therefractive index of the first, second, and third packaging layers 160,162, 164, even though this may entail a degradation of the performance.It is further possible for there to be present, above the portion ofsecond dielectric layer 32 that overlies the anode region 12, one ormore additional layers (not shown), arranged between said portion andthe first packaging layer 160. For example, the first packaging layer160 may be arranged on a nitride layer (not shown), in direct contacttherewith, this nitride layer being arranged over the second dielectriclayer 32, with which it is in direct contact. In this case, the nitridelayer and the second dielectric layer 32 may form an optimizedanti-reflective structure for increase of transmission in the infrared.

The third absorption layer 164 may be constrained to the firstabsorption layer 160 in a way different from bonding, in which case thesecond packaging layer 162 may be absent. For example, the thirdpackaging layer 164 may be formed by being poured in the liquid formover the first packaging layer 160, and then hardened. It is furtherpossible for the third packaging layer 164 to be formed by a paint laidon the first absorption layer 160.

It is possible for the photodiodes to be such that the array 99 does notoperate as SiPM. For example, instead of the cathode metallization 42,shared between the SPADs 1, a plurality of cathode metallizations may bepresent, each of which is associated to a corresponding SPAD 1.

Finally, it is possible for the array 99 to be packaged with a packagingdifferent from SMD packaging.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1. An optoelectronic device, comprising: a first array of Geiger-modeavalanche photodiodes formed in a die; and a packaging structure, whichincludes: a first external dielectric region formed by a first materialconfigured to absorb secondary photons from the photodiodes in a firstwavelength range and transmit radiation having a wavelength that fallsin a pass-band with a second wavelength range that is higher than thefirst wavelength range, at least part of said pass-band includinginfrared wavelengths; an internal dielectric structure positionedbetween the first array and the first external dielectric region andformed by one or more second materials that substantially transmitradiation having a wavelength that falls in either of said first andsecond wavelength range.
 2. The optoelectronic device according to claim1, wherein said internal dielectric region comprises a first layer madeof an epoxy resin and arranged on the die.
 3. The optoelectronic deviceaccording to claim 2, wherein the first layer surrounds the dielaterally.
 4. The optoelectronic device according to claim 2, whereinsaid internal dielectric region comprises a second layer that bonds thefirst layer and the first external dielectric region together.
 5. Theoptoelectronic device according to claim 1, wherein said first externaldielectric region is made of a polymeric plastic material.
 6. Theoptoelectronic device according to claim 5, wherein said first externaldielectric region is made of allyl diglycol carbonate or polyallyldiglycol carbonate.
 7. The optoelectronic device according to claim 1,wherein said die comprises a semiconductor body having a front surfaceand including a cathode region of a first type of conductivity; andwherein each photodiode comprises: a respective anode region of a secondtype of conductivity, which extends within the cathode region; a lateralinsulation region extending through the body starting from the frontsurface and surrounding the respective anode region and a correspondingpart of the cathode region, said lateral insulation region including abarrier region and an insulating region, which surrounds the barrierregion, said barrier region being configured to absorb or reflectradiation.
 8. The optoelectronic device according to claim 7, whereinsaid anode region extends into the semiconductor body from the frontsurface.
 9. The optoelectronic device according to claim 7, wherein saiddie comprises a passivation region, which extends over the frontsurface, in contact with the semiconductor body; and wherein saidinternal dielectric structure extends in contact with said passivationregion.
 10. The optoelectronic device according to claim 9, wherein saidpassivation region has a refractive index, which, in said stop-band andin said pass-band, falls in said interval.
 11. The optoelectronic deviceaccording to claim 1, further comprising: a second array of Geiger-modeavalanche photodiodes; and a second external dielectric region overlyingsaid second array and made of a third material having a differentpass-band than the first material.
 12. The optoelectronic deviceaccording to claim 1, wherein said one or more second materials haverefractive indices that fall in an interval having a width of 0.4; andsaid first external dielectric region has a refractive index with a realpart that falls within said interval.
 13. A photon-detection systemcomprising: an optoelectronic device that includes: a first array ofGeiger-mode avalanche photodiodes, said first array being formed in adie and a packaging structure that includes an internal dielectricstructure directly on the die and overlying said photodiodes; and afirst external dielectric region directly on the internal dielectricstructure; a light source optically coupled to the first array; and aprocessing unit electrically coupled to the first array, wherein: saidfirst external dielectric region is formed by a first material thatabsorbs radiation having a wavelength that falls in a stop-band with afirst wavelength range and transmits radiation having a wavelength thatfalls in a pass-band with a second wavelength range that is higher thanthe first wavelength range, at least part of said pass-band includinginfrared wavelengths; said internal dielectric structure is formed byone or more second materials that substantially transmit radiationhaving a wavelength that falls in said stop-band and in said pass-band.14. The photon-detection system according to claim 13, wherein saidprocessing unit comprises a microcontroller unit, or else adiscriminator and a counter electrically coupled together.
 15. Thephoton-detection system according to claim 13, wherein said diecomprises a semiconductor body having a front surface and forming acathode region of a first type of conductivity; and wherein eachphotodiode comprises: a respective anode region of a second type ofconductivity, which extends within the cathode region; a lateralinsulation region extending through the body starting from the frontsurface and surrounding the respective anode region and a correspondingpart of the cathode region, said lateral insulation region including abarrier region and an insulating region, which surrounds the barrierregion, said barrier region being configured to absorb or reflectradiation.
 16. The photon-detection system according to claim 13,wherein the optoelectronic device includes: a second array ofGeiger-mode avalanche photodiodes; and a second external dielectricregion overlying said second array and made of a third material having adifferent pass-band than the first material.
 17. An optoelectronicdevice, comprising: a first array of Geiger-mode avalanche photodiodesconfigured to detect incoming photons and emit secondary photons thathave a wavelength in a first wavelength range between 400 nm and 600 nm;a packaging structure, which includes: a first external dielectricregion formed by a first material that absorbs the secondary photons inthe first wavelength range and transmits radiation having a wavelengththat falls in a pass-band with a second wavelength range that is higherthan the first wavelength range, at least part of said pass-bandincluding infrared wavelengths; an internal dielectric structurepositioned between the photodiodes and the first external dielectricregion and formed by one or more second materials that substantiallytransmit radiation having a wavelength that falls in either of saidfirst and second wavelength ranges.
 18. The optoelectronic deviceaccording to claim 17, comprising a semiconductor body having a frontsurface and forming a cathode region of a first type of conductivity;and wherein each photodiode comprises: a respective anode region of asecond type of conductivity, which extends within the cathode region; alateral insulation region extending through the body starting from thefront surface and surrounding the respective anode region and acorresponding part of the cathode region, said lateral insulation regionincluding a barrier region and an insulating region, which surrounds thebarrier region, said barrier region being configured to absorb orreflect radiation.
 19. The optoelectronic device according to claim 18,comprising a passivation region, which extends over the front surface,in contact with the semiconductor body; and wherein said internaldielectric structure extends in contact with said passivation region andsaid passivation region, first external dielectric region, and internaldielectric structure each have a refractive index that falls in aninterval having a width of 0.4.
 20. The optoelectronic device accordingto claim 17, further comprising: a second array of Geiger-mode avalanchephotodiodes; and a second external dielectric region overlying saidsecond array and made of a third material having a different pass-bandthan the first material.